Protective coatings for gas tank containers are subjected to external and environmental abuse as well as internal solvent degradation. Externally, a gas tank must endure abrasion from gravel and other road debris as well salt degradation. Accordingly, the exterior coating is generally a zinc containing compound having a relatively hard surface. Although such a surface must be adaptive to welding and/or soldering additives, prior art coatings tended to decrease weldability.
Prior art resin coatings, such as the coatings disclosed in the U.S. Pat. No. 4,391,855 to Geeck issued July 5, 1983 are resin coatings including cross-linking agents. A secondary resin is added, such as a melamine formaldehyde type cross-linker. Such cross-linking agents required extended periods of time for cure. Longer term baking or air drying is required. While the use of lower molecular weight resins alone provide good adhesion to the metal substrate and good formability, such resins alone are too soft for exterior gas tank protection.
Gas tanks generally have a lead and tin plate on the surface. Such surfaces are hard to coat, making adhesion another critical factor in formulating a corrosion resistant coating. As previously stated, lower molecular weight resins alone provide good adhesion to metal substrates and good formability but are too soft for exterior gas tank protection. At the other extreme, high molecular weight resins do not provide sufficient adhesion and do not provide sufficient formability.
The interior of a gas tank does not require the same protective characteristics as the exterior of the gas tank. However, corrosion resistance, formability, and adhesion still remain critical factors. Lower molecular weight resins provide good adhesion and formability. However, the interior of the tank is exposed to unleaded gasoline and/or other fuel and gasoline additives. Solvents such as these attack the lower molecular weight resin surface causing swelling and leaching of the lead from the metal substrate. Leaching results in contamination and eventual failure of engine components.